Done!
I've just received the registration email and gone through a different route:
To insert your license key, please follow the instructions below:
1. Highlight the license key above, then right-click it and select "Copy".
2. Open your avast! Free Antivirus controls and select the "Settings" tab.
3. Next, click the "Registration" tab.
4. Click the arrow next to "Offline registration" to expand this option.
5. Click the "Insert the license key" button.
6. Right-click in the empty Registration box and select "Paste".
7. Click "OK" – your license key is now inserted and your avast! antivirus software can now be used free of charge for a further 12 months.
Thanks again